1. Technical Field
The present invention relates generally to switches. More particularly, it relates to microfabricated electromechanical switches having a single pole double throw configuration with the ability to latch.
2. Description of Related Art
Switch networks are found in many systems applications. For example, in satellite systems, switch networks are essential for routing matrices and redundancy systems. Future satellite systems will not only require larger switch routing networks, but also increased functionality for network-centric operations. These new capabilities will include spacecraft reconfiguration for beam switching, beam shaping, and frequency agility. Thus, it is expected that satellites will require an increasing number of switches in their payloads.
In many cases, these switches need to be latching, that is, once they are actuated they will remain in a desired state even after the actuation energy source is removed. Some of the applications where latching switches are important are ultra-reliable networks where power interruptions could create a problem, such as satellite or Unmanned Air Vehicles, or networks where supplied power is limited, like in small mobile platforms that run on batteries. Current latching switch technology typically relies on magnetic or motor drives to change switch states. These switches, typically fabricated using coaxial conductors or metallic waveguides, generally work very well. However, most of the applications listed above would benefit from size and weight reduction since the mechanical latching switches currently in use tend to be larger and heavier than desired. Semiconductor switches, such as made using PIN diodes and FET switches, are small, but they typically cannot latch in multiple states without a constant energy source.
Radio Frequency (RF) Micro Electro-Mechanical System (MEMS) switches are known in the art to have small size and weight and are also known to provide desirable performance in the radio frequency and microwave spectrums. Several types of MEMS switches are well-known in the art. For example, U.S. Pat. No. 5,121,089 issued Jun. 9, 1992 to Larson discloses a microwave MEMS switch. The Larson MEMS switch utilizes an armature design. One end of a metal armature is affixed to an output line, and the other end of the armature rests above an input line. The armature is electrically isolated from the input line when the switch is in an open position. When a voltage is applied to an electrode below the armature, the armature is pulled downward and contacts the input line. This creates a conducting path between the input line and the output line through the metal armature. This switch requires a constant voltage to maintain the switch in a closed state.
As another example, U.S. Pat. No. 6,046,659 of Loo et al. discloses methods for the design and fabrication of non-latching single pole single throw MEMS switches. U.S. Pat. No. 6,046,659 is incorporated herein by reference in its entirety. FIG. 1 shows a top view of a MEMS switch 10 according to Loo et al., which provides single pole single throw switching between an input line 20 and an output line 18 when electrically actuated with a DC voltage.
FIGS. 2A and 2B are side-elevational views of the MEMS switch 10. FIG. 2A shows the switch 10 in the open position and FIG. 2B shows the switch 10 in the closed position. Beam structural material 26 is connected to a substrate 14 through a fixed anchor via 32. A suspended armature bias electrode 30 is nested within the structural material 26 and electrically accessed through a bias line 38 at an armature bias pad 34. A conducting transmission line 28 is at the free end of the beam structural layer 26 and is electrically isolated from the suspended armature bias electrode 30 by the dielectric structural layer 26. Contact dimples 24 of the transmission line 28 extend through and below the structural layer 26 and define the areas of metal contact to the input and output lines 20 and 18, respectively. A substrate bias electrode 22 is below a suspended armature bias electrode 30 on the surface of the substrate 14. When a voltage is applied between the suspended armature bias electrode 30 and the substrate bias electrode 22, an electrostatic attractive force will pull the suspended armature bias electrode 30 as well as the attached armature 16 towards the substrate bias electrode 22. The contact dimples 24 touch the input line 20 and the output line 18, so the conducting transmission line 28 bridges the gap between the input line 20 and the output line 18, thereby closing the MEM switch.
Loo et al. generally describe a surface micromachined device. That is, layers are deposited on top of a substrate, and then one or more of the layers is etched away to release the moving parts of the switch 10. As described in Loo et al., the parts of the switch generally comprise gold (or gold alloys) for the switch contacts, silicon dioxide for the one or more layers etched away (i.e., the sacrificial layers), and silicon nitride for the beam structural layer. However, as discussed in additional detail below, switches fabricated according to Loo et al. may exhibit some problems.
The switches fabricated according to Loo et al. are typically fabricated with one layer deposited on the next. With such fabrication, any pattern of one layer may get transferred to each subsequent layer. The dimensions of the switch dielectric and metal layers are typically thin enough that the transferred copies of the initial metal layer pattern (for example, the pattern of the substrate bias electrode 22) appear even at the top nitride layer of the dielectric structural layer 26. Therefore, as layers of SiO2 and Si3N4 are deposited on top of the bottom metal layer, these dielectric layers may wrap around the bottom metal structures, in particular, the substrate bias electrode 22. In some cases, after the sacrificial silicon dioxide was etched away, the remaining silicon nitride formed a lid that covered the substrate bias electrode 22 when the switch 10 was closed.
The formation of the silicon nitride “lid” is shown in FIG. 5, which illustrates the dielectric structural layer 26 wrapping around the bias electrode 22 disposed on the substrate 14. Because of the tightness of the fit of this nitride “lid” over the bottom electrode, there may be great deal of friction between the lid and the substrate bias electrode 22 when the switch 10 is opened and closed. The friction of the lid may depend upon post-processing used to etch away the sacrificial layer. The lid may be made to fit more loosely over the substrate bias electrode 22 by etching longer, so that some of the silicon nitride is etched away. However, in some cases, the switch 10 would close upon actuation and not open upon the removal of the actuating voltage. Therefore, as indicated above, control of the design of the switch and the processes used to fabricate the switch may be required to avoid the friction problems in the prior art switch according to Loo et al.
An example of a latching micro switch is described in U.S. Pat. No. 6,496,612 issued Dec. 17, 2002 to Ruan et al. Ruan et al. describe a switch having a cantilever to switch between an open state and a closed state. To operate as a latching switch, a permanent magnet is used to maintain the cantilever in an open state or a closed state. However, the use of a permanent magnet may result in a switch that is bigger and/or heavier than desired.
Another example of a latching switch is described by Xi-Qing Sun, K. R. Farmer and W. N. Carr in “A Bistable Micro Relay Based on Two-Segment Multimorph Cantilever Actuators,” The Eleventh Annual International Workshop on Micto-electro Mechanical Systems, 1998, MEMS 98 Proceedings, Jan. 25-29, 1998, pp. 154-159. Sun et al. describe a latching switch mechanism that uses two metals to create stresses in opposite directions along a cantilever beam. RF contacts can be moved by controlling the stress on the two segments electrostatically to lengthen or shorten the length of the cantilever along the substrate so that the contact can be moved from one RF line to another. The fabrication of the switch disclosed by Sun et al. may be complicated since two different metals are required. Further, the switch disclosed by Sun et al. requires two independent control voltages to move the switch.
Still another example of a single pole double throw switch is described in U.S. Pat. No. 6,440,767 B1, issued Aug. 27, 2002 to Loo et al. This switch is similar to that described above in U.S. Pat. No. 6,046,659, except that two armatures are used to provide the single pole double throw switching action. As such, the switch may exhibit the same problems described above in regard to the switch disclosed in U.S. Pat. No. 6,046,659.
Therefore, there is a need in the art for a small, lightweight latching switch that does not require an external voltage or magnetic source to stay latched in a selected state.